A hydrogel is a hydrated, crosslinked polymeric system that contains water in an equilibrium state. Hydrogel contact lenses offer certain oxygen permeability as well as desirable biocompatibility and comfort. Conventional hydrogel materials (e.g. 2-hydroxyethylmethacrylate, HEMA) by themselves have poor oxygen permeability and they transport oxygen to the eye through the absorbed waterphase, but on the other hand due to the hydrophilic nature of the monomer, HEMA based lenses has better surface wettabilities which is very favorable for ophthalmic application. Silicone-hydrogels are used to make medical devices including implants and contact lenses of daily disposable and extended wear, as well as a group of rigid gas permeable lenses due to their relatively high oxygen permeability. Traditional siloxane monomers/polymers are hydrophobic and lenses made with them often require additional treatment for more hydrophilic surface. Many efforts were put into overcoming the hydrophobicity of the silicone materials.
Now silicone-hydrogels with the comfort of soft contact lenses and significantly higher oxygen permeability overcame the obstacles for periods of wear beyond conventional hydrogels and were revolutionary in the field of optometry. The following patents describe silicone-hydrogels for use in contact lenses.
U.S. Pat. No. 4,260,725 to Bausch & Lomb Inc., describes a water absorbing, soft, hydrophilic, flexible, hydrolytically stable, biologically inert contact lens with the capability of transporting oxygen sufficiently to meet the requirements of the human cornea comprising a polysiloxane which is α,ω terminally bonded through divalent hydrocarbon groups to polymerizably activated unsaturated groups and which contain hydrophilic side chains.
Water has low oxygen permeability, also called the Dk value, which may be expressed in Barrer, wherein 1 Barrer=10−11 (cm3 O2) cm cm−2 s−1 mmHg−1, “cm3 O2” is at a quantity of oxygen at standard temperature and pressure; “cm” represents the thickness of the material; and “cm−2” is the reciprocal of the surface area of that material. The Dk of water is 80 Barrer. Upon exposure to atmospheric air for long periods, these lenses are slowly dehydrated and the amount of oxygen transported to the cornea is reduced. Eye irritation, redness and other corneal complications can result and hence restrict use of the lenses to limited periods of wear.
U.S. Pat. No. 5,352,714 also to Bausch & Lomb Inc. describes silicone-containing hydrogels with enhanced wettability comprising a silicone-containing monomer, hydrophilic monomers, and a relatively non-polar ring-containing monomer able to be converted to a highly polar amino acid upon hydration.
U.S. Pat. No. 5,998,498 to Johnson & Johnson Vision Products describes a silicone hydrogel prepared by curing a reaction mixture comprising a silicone-containing monomer having the following structure:
wherein R51 is H or CH3, q is 1 or 2 and for each q, R52, R53 and R54 are independently ethyl, methyl, benzyl, phenyl or a monovalent siloxane chain comprising from 1 to 100 repeating Si—O units, p is 1 to 10, r=(3−q), X is 0 or NR55, where R55 is H or a monovalent alkyl group with 1 to 4 carbons, a is 0 or 1, and L is a divalent linking group which preferably comprises from 2 to 5 carbons, which may also optionally comprise ether or hydroxyl groups, for example, a polyethylene glycol chain.
U.S. Pat. No. 6,867,245 to Asahikasei Aime Co. describes a soft contact lens, and provides a contact lens that shows small and stable contact angle to water at its surface in water as well as in air, little deposition in wearing, high oxygen permeability, no adhesion of lens to a cornea, and superior extended-wearing characteristics. It describes a hydrogel soft contact lens, which has a contact angle at a lens surface in a range of 10-50° by the captive bubble method in water and 30-90° by the sessile drop method in air, oxygen permeability of not less than 30 and water content of not less than 5%, and also a hydrogel soft contact lens consisting of a polymer comprising a hydrophilic siloxanyl monomer shown by a specified general formula. This patent discloses copolymers of hydrophilic siloxane with amide-group containing monomers that are stated as being useful materials for contact lenses. The polymer comprises hydrophilic amide-group containing siloxanyl methacrylate, a siloxanyl methacrylate (tris[trimethylsiloxy]silylpropylmethacrylate, abbreviated as TRIS) including a hydrophilic polyether modified siloxanyl alkyl methacrylate and a crosslinkable monomer.
U.S. Pat. No. 6,013,711 to the CK Witco Corporation describes a method for improving the miscibility of the lower molecular weight unsaturated siloxane-polyether copolymers with the α,ω-divinylpolysiloxanes without loss of storage stability, or delay of cure at the vulcanization temperature, or loss of permanent hydrophilicity or other desirable features of the cured polysiloxane. The compositions comprise one or more α,ω-divinylpolysiloxanes, unsaturated polysiloxane-polyether copolymers having from 2 to 5 silicon atoms per molecule, which are preferably trisiloxanes, and a compatibilizing additive. The permanently hydrophilic, rapidly wettable polysiloxane compositions yield static water contact angles <50° and dynamic advancing contact angles of less than about 100.
U.S. Pat. No. 6,207,782 to Crompton Corporation discloses acrylated hydrophilic polysiloxane monomers and polymers and their copolymers with acrylate/methacrylate comonomers and their emulsions for personal care, textile and coating applications. The acrylated siloxanes are represented by formula (a):[R3SiO1/2]m[O1/2SiR2O1/2]n[SiO3/2R]o[SiO4/2]p  (a)wherein R is selected from the R1 and P, wherein each R1 can be the same or different and each is a monovalent hydrocarbon group; each P is R3[O(CbH2bO)zCOCR4═CH2]g wherein, R3 is a polyvalent organic moiety, which may be hydroxyl substituted alkylene, g is the valency of R3 minus 1, R4 is hydrogen or methyl; b=2 to 4, preferably 2 to 3; z=1 to 1000, preferably 3 to 30; and m+n+p+o=1 to 100, preferably 2 to 20, at least one R is P; n=1 to 100; when o is not zero n/o<10:1; when p is not zero n/p<10:1; and m=0 to 10. A preferred acrylated siloxane in the patent is of the Formula (b):QR12Si[OSiR12]x[O—SiR1P]yOSiR12Q  (b)wherein x, and y can be 0 or an integer, preferably each x and y are from 0 to 100, most preferably 0 to 25; Q can be R1 or P, with the proviso that the average acrylate functionality is greater than 1 unsaturated groups per molecule with the preferred embodiment having y=0 and Q=P.
Conventionally, silicone-hydrogels are made by polymerizing the acrylate or methacrylate functionalized silicone monomer with hydrogel (hydrophilic) monomers, such as hydroxyethyl methacrylate (HEMA), N-Vinylpyrrolidone (NVP) and other monomers such as methyl methacrylic acid (MA), Dimethylacrylamide (DMA), etc, in the presence of crosslinker and thermal radical initiator or photoinitiators. Crosslinking agents generally have two or more reactive functional groups at different sites of the molecule. Typically, these sites contain polymerizable ethylenic unsaturation groups. During curing, they form a covalent bond with two different polymer chains and form a stable three-dimensional network to improve the strength of the polymer. Crosslinking agents conventionally used in contact lenses include ethylene glycol dimethacrylate and trimethyloylpropane trimethacrylate (about 0.1 to 2 wt %). Other useful crosslinking agents include diethyleneglycol dimethacrylate, bisphenol A dimethacrylate, diglycidyl bisphenol A dimethacrylate and dimethacrylate-terminated polyethylene glycol and reactive linear polyether modified silicones.
Generally, silicone hydrogel contact lens materials are made using hydrophobic mono-functional silicone monomer (such as TRIS) and/or difunctional hydrophilic silicone monomer followed by secondary surface treatment. The functionality of the siloxane monomers/polymers can affect the resulting silicone hydrogel's modulus.
The state of this art for soft contact lenses, including the silicone-based materials described in the above mentioned patents, still possess major shortfalls like sub-optimal surface wettability and lipid deposition. Silicone is an extremely hydrophobic material and difficult to modify to form a silicone-based material that is truly water soluble. U.S. Pat. No. 7,781,558 titled “Hydrophilic Siloxanyl Monomers With Pendant Polymerizable Groups” and U.S. Patent Publication No. 2008/0076898 entitled “Water Soluble Silicone Macromonomers for Ophthalmic Materials” describe incorporating ionic moieties into a polysiloxane side chain to impart hydrophilicity to the molecule and claim the formulation can possibly extracted by water. Still, the silicone or siloxane-based materials from the above patents are not fully water soluble.
Some silicone hydrogel materials are water dispersible and can be copolymerized with water-soluble organic monomers. However, materials that are water dispersible may not necessarily be fully water soluble. Non-fully water soluble monomers will likely require a non-water or non-aqueous solvent, or a compatibilizer such as a surfactant to ensure copolymerization. The solvent and compatibilizer must be removed, which increases manufacturing costs. Additionally, if the modified silicone monomer is not fully water-soluble, the silicone segment of the resulting polymer will aggregate into micelles in an aqueous medium, e.g., where contact lenses are stored and worn, in order to reduce free energy. In such conditions, silicone domains will have a minimum effect in improving oxygen permeability as the oxygen molecules will still need to transport through the aqueous medium, which has much lower oxygen solubility.
In an effort to overcome these drawbacks, current technology uses either expensive secondary surface treatments called “plasma oxidation” or use internal wetting agents. at relatively high manufacturing cost. Hence there remains a need for hydrophilic silicone monomers with inherently advantageous wettability and oxygen permeability that can be used to make contact lenses without the drawbacks and expensive surface treatments necessary with the silicone containing materials of the present art. When the material is highly water soluble, the formulation can be processed in water, this eliminates the use of organic solvent, compatilizer, surfactant in formulation and during extraction.